Multichannel high-speed data transfer

ABSTRACT

A digital mobile communication system has a high-speed non-transparent data connection between a transmitting and a receiving party (MS, TAF). For the data connection, parallel subchannels (ch1-chn), corresponding in number to the nominal data transfer rate, have been allocated on the radio interface. A radio link protocol (RLP) is responsible for transmitting data over the radio interface, and for acknowledging correct data frames and for retransmitting defective data frames. A transmission buffer (63) buffers the data frames to be transmitted and stores the data frames transmitted until it receives an acknowledgement of successful reception. In order to reduce interference and power consumption, user data is transmitted by using as many of the allocated subchannels as required by the actual user data rate at any one time. On the other allocated subchannels, transmission is interrupted or discontinuous transmission is activated.

FIELD OF THE INVENTION

The present invention relates to high-speed multichannel data services(HSCSD) on a radio interface of a mobile communication system.

BACKGROUND OF THE INVENTION

In mobile telecommunication systems of the time division multiple access(TDMA) type, time-division communication takes place on the radio pathin successive TDMA frames, each of which consists of several time slots.In each time slot, a short information packet is sent as a radiofrequency burst which has a finite duration and which consists of a setof modulated bits. The time slots are mainly used for transmittingcontrol channels and traffic channels. On the traffic channels, speechand data are transmitted. On the control channels, signalling between abase station and mobile subscriber stations is carried out. An exampleof a TDMA radio system is the Pan-European mobile communication systemGSM (Global System for Mobile Communications).

In conventional TDMA systems, each mobile station is assigned onetraffic channel for data or speech transmission. Thus, the GSM system,for instance, may have up to eight parallel connections to differentmobile stations on a same carrier wave. The maximum data transfer rateon one traffic channel is restricted to a relatively low level accordingto the available bandwidth and the channel coding and error correctionused in the transmission, for example in the GSM system to 12 kbit/s, 6kbit/s or 3.6 kbit/s.

A digital mobile communication system typically uses several connectiontypes which can be divided into two categories: transparent andnon-transparent connections. On a transparent connection, data istransferred through a traffic channel of the mobile communication systemin a transparent way, which means that error correction on the radiopath is carried out by employing channel coding only. In the GSM systemthe channel coding is Forward Error Correction (FEC). A non-transparentconnection uses, in addition to channel coding, an additional protocolin which the data transmission over the traffic channel is repeated incase the data was not received correctly at the other end. In the GSMsystem, this communication protocol is Radio Link Protocol (RLP), usedbetween a terminal adaptor of a mobile station MS and an interworkingfunction IWF, which is typically at a mobile services switching centerMSC. The RLP is a balanced (HDLC type) data transfer protocol having aframe structure. Error correction by the RLP is based on retransmissionof frames corrupted on the traffic channel. There is another protocol,Layer 2 Relay (L2R), above the RLP. In the present patent application,the functional part of the TAF or IWF carrying out these protocols isreferred to as an L2R/RLP unit.

In a normal data transfer state, the L2R/RLP unit packs user data into200-bit long protocol data units (PDU), which are transmitted in 240-bitRLP frames over the radio interface to a second L2R/RLP unit. If thereis no data or other information to be transferred between the twoL2R/RLP units, discontinuous transmission (DTX) may be applied. DTXrefers to a method reducing transmission on the radio path to a minimum(i.e. interrupt the transmission) during pauses in the data transfer.The aim is to reduce the power consumption of the transmitter, a verysignificant matter for the mobile stations, as well as the overallinterference level on the radio path, which has an effect on the systemcapacity. The DTX operates independently for the uplink and downlinkdirections. The mobile communication network may either allow orprohibit the use of DTX.

In normal L2R/RLP operation, the PDUs are possibly filled onlypartially, because the application may limit the maximum user rate belowthe maximum rate on a traffic channel. The PDUs may be full if theactual user data rate on the terminal interface is high enough or if,due to delays caused by re-transmission or some other congestion, theL2R/RLP buffer has enough user data to fill one PDU completely. It isalso possible, depending on the implementation, to prefer full PDUs topartially full PDUs. This can be accomplished by using a timer or acounter to slightly delay the building of a PDU until there is enoughdata available (e.g. in a buffer) for a full PDU to be built, or untilthe building of a PDU, albeit only a partially full PDU, cannot bedelayed any longer. The timer may have a typical value in the order of20 ms, in other words the repetition period of TDMA frames. The timervalue should be relatively short in order not to introduce additionaldata transmission delays.

However, the data rates of present-day mobile communication networks arenot sufficient for the new, high-speed data services. A solution isproposed for introducing higher data rates for mobile communicationsystems in the applicant's co-pending PCT application WO95/31878,unpublished on the priority date of the present application: two or moreparallel traffic channels (subchannels) are used on the radio path forone high-speed data connection. The high-speed data signal isdistributed to the parallel subchannels at the transmitting end for thetransmission over the radio path, and then combined at the receivingend. In this manner, it is possible to provide data transfer serviceswhich, depending on the number of allocated traffic channels, have atransfer rate up to 8 times the conventional data rate. In the GSMsystem, for example, the total user data transfer rate 19.2 kbit/s isobtained with two parallel subchannels. This principle is also referredto as a multi-slot channel technique. High-speed data service thusobtained are referred to as HSCSD (High Speed Circuit Switched Data)services.

If the user data rate in a HSCSD service is lower than the maximumcapacity of the radio link protocol, partially filled PDUs may be builtand transmitted over the radio interface in the RLP frames. It is alsopossible that some subchannels carry full PDUs in their RLP frames andsome subchannels occasionally carry partially full or empty PDUs intheir RLP frames.

For the mobile station, this inefficient use of transmission capacitymay lead to an unnecessarily high power consumption, heating of RF andother elements, and possibly a more complex scheduling of reception,transmission and neighbour cell monitoring than necessary for the actualuser data rate.

For the radio interface, this leads to an increased interference, eitherin the uplink or the downlink direction, or both. For the base stationand the IWF, reducing complexity is not that much of an issue as for theMS.

The prior art DTX implementation, and the slightly delayed building ofPDUs can relief the situation somewhat but not completely. The DTX ismainly used mainly in cases when there is nothing at all to betransmitted. When some data has to be transmitted (at a low rate), thetransmission requiring only a fraction of the allocated bandwidth, theconventional DTX does not suffice.

DISCLOSURE OF THE INVENTION

An object of the present invention is a discontinuous transmissionsuitable for multichannel high-speed data links.

This object is achieved by a method for high-speed data transfer in adigital mobile communication system, the method comprising the steps ofestablishing a non-transparent data connection having a number ofparallel subchannels allocated on the radio interface, said number beingdetermined by a specific maximum transfer capacity; receiving user datafrom a terminal interface at a varying user data rate; transmitting userdata over the non-transparent data connection in data frames byemploying a communication protocol which acknowledges data framesreceived correctly and retransmits defective data frames; buffering dataframes to be transmitted in a transmission buffer; storing the dataframe transmitted in the transmission buffer for a possibleretransmission until an acknowledgement is received from the receivingend. The method is characterized by determining the actual user datarate on the terminal interface; determining a minimum number ofsubchannels, said number being determined by the actual user data rate;transmitting user data in data frames only via specific subchannelscorresponding in number to said minimum number of subchannels;interrupting transmission or activating discontinuous transmission oneach surplus subchannel allocated to the connection; monitoring filllevel of the transmission buffer; continuing transmission ordeactivating discontinuous transmission on at least one of said surplussubchannels if the transmission buffer fill level reaches a firstthreshold value; interrupting transmission or reactivating discontinuoustransmission on at least one of said surplus subchannels if thetransmission buffer fill level decreases to a second threshold level.

The basic concept of the present invention is to transmit frames of theradio link protocol (RLP) selectively only via specific subchannels incases the maximum data transfer capacity allocated to the data link isnot required. This is important because interleaving on the radiointerface spreads an RLP frame over several TDMA frames. If the RLPframes were transmitted over arbitrarily selected subchannels withoutany consistency, many or maybe all allocated subchannels wouldconstantly be "active". According to the present invention,transmissions are concentrated on specific subchannels only, whereas theother subchannels allocated to the connection carry no transmission atall, or they employ subchannel-specific DTX. The direct benefits of alower number of active subchannels include reduced transmitter powerconsumption, less temperature problems and a simpler timing ofreception, transmission, and measuring neighbouring cells. In addition,as the number of unnecessary transmissions on the radio interface islower, the interference level in the mobile communication network willbe lower.

The required minimum number of subchannels can be determined bymonitoring the data flow into the transmission buffer, i.e. the actualuser data rate. Furthermore, by monitoring the amount of buffered datait is possible to determine whether more active subchannels will berequired than said minimum number, and to dynamically increase anddecrease the number of subchannels in use. Data amount mentioned abovecan be represented e.g. data rate, buffer status, number of PDUs or RLPframes etc. Determining the number of subchannels used, and weightingsfor different subchannels may be based on various mathematical andstatistical variables of the amount of input and buffered data, suchvariables being for example instantaneous value, a fixed average, amoving average, or some other statistical variable (geometric mean,median, etc). This allows the control process to react in a controlledfashion to sudden, slow, temporary or long term changes in thetransmission capacity requirements and availability. Such changes may becaused by, for example, handover, bad coverage (temporary or long term),a data transfer request, requested retransmission of corrupted data,allocation of new subchannels to the connection, removal of subchannelsfrom the connection, and changes in the radio interface channel coding.

There are different ways to choose the subchannels on which datatransfer will be continued. One of the embodiments of the inventionutilizes a subchannel preference list which organizes the subchannelsaccording to principles such as: (1) an order based on the position ofsubchannels in a TDMA frame, (2) a predetermined order which depends onthe total number of subchannels on the connection, (3) an order to benegotiated during the connection, or (4) an arbitrary order. Even anarbitrary order is advantageous if maintained the same for the durationof several TDMA frames.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described in greater detail bymeans of the preferred embodiments, with reference to the accompanyingdrawings, in which

FIG. 1 illustrates a part of a mobile communication system to which thepresent invention can be applied on a single channel non-transparentconnection,

FIG. 2 is a block diagram illustrating the functional units of a singlechannel non-transparent GSM traffic channel on different protocollevels,

FIG. 3 shows a L2R PDU,

FIG. 4 shows an RLP frame,

FIG. 5 illustrates a part of a GSM mobile communication system to whichthe invention can be applied on a multichannel non-transparentconnection,

FIG. 6 is a block diagram illustration of an arrangement for an HSCSDconnection in accordance with the invention.

PREFERRED EMBODIMENTS OF THE INVENTION

The present invention can be applied to a high-speed data transmissionin digital mobile communication systems utilizing various multipleaccess methods, such as TDMA or CDMA. In different multiple accessmethods the physical concept of traffic channel varies, being primarilydefined by a time slot in TDMA systems, a spreading code in CDMAsystems, a radio channel in FDMA systems, a combination thereof, etc.The basic concept of the present invention is, however, independent ofthe type of the traffic channel and the multiple access method used.

The present invention can be used in all digital data transfer systemson a non-transparent data link comprising several parallel subchannels(e.g. multi-slot access).

The present invention is particularly well suited to data transferapplications in digital TDMA mobile communication systems, such as thePan-European digital mobile communication system GSM, DCS1800 (DigitalCommunication System), a mobile communication system according to theEIA/TIA Interim Standard IS/41.3, etc. Below, the invention will bedescribed by using the GSM system as an example but without restrictingthe invention to it. FIG. 1 very briefly shows the basic structure ofthe GSM system, not paying closer attention to its characteristics orother aspects of the system. For a more detailed description of the GSMsystem, the GSM recommendations and "The GSM System for MobileCommunications", M. Mouly & M. Pautet, Palaiseau, France, 1992,ISBN:2-9507190-0-7, are referred to.

A mobile services switching centre MSC handles the connecting ofincoming and outgoing calls. It performs functions similar to those ofan exchange of a public switched telephone network (PSTN). In additionto these, it also performs functions characteristic of mobilecommunications only, such as subscriber location management, jointlywith the subscriber registers (not shown) of the network. The mobilestations MS are connected to the center MSC by base station systems BSS.The base station system BSS consists of a base station controller BSCand base stations BTS.

The GSM system is a time division multiple access (TDMA) system in whichtime-division traffic takes place on the radio path in successive TDMAframes each of which consists of several time slots. In each time slot,a short information packet is sent as a radio frequency burst which hasa finite duration and which consists of a set of modulated bits. Thetime slots are mainly used for transmitting control channels and trafficchannels. On the traffic channels, speech and data are transmitted. Onthe control channels, signalling between a base station and mobilesubscriber stations is carried out. Channel structures used on the radiointerface of the GSM system are defined in closer detail in the GSMrecommendation 05.02. In operating according to the recommendation, atthe beginning of a call a mobile station MS is assigned a time slot froma carrier wave as a traffic channel (Single Slot Access). The MSsynchronizes to this time slot to transmit and receive radio frequencybursts.

In the GSM system, a data link is established between a mobile stationMS network terminal TAF (Terminal Adaptation Function) 31 and a networkadaptor IWF (Interworking Function) 41 in the fixed network-(usually atthe MSC). The data link is a circuit-switched connection which reservesone (or more) traffic channel(s) from the radio interface for theduration of the connection. In the GSM network, the data link in datatransfer is a V.110 rate adapted, V.24 interface compatible, UDI codeddigital Full Duplex connection. The V.110 connection is originally adigital transmission channel developed for ISDN (Integrated ServicesDigital Network), specified in the recommendation CCITT Blue Book V.110.The terminal adaptor TAF adapts a data terminal TE connected to the MSfor the V.110 connection which in FIG. 1 is established over acircuit-switched connection using traffic channel ch0. The networkadaptor IWF adapts the V.110 connection to another V.110 network such asan ISDN or another GSM network, or to another transit network, e.g. thepublic switched telephone network PSTN.

In addition, the traffic channel employs channel coding FEC (ForwardError Correction) with the aim of reducing the effect of transmissionerrors on the radio path. The GSM system employs convolution codingaccording to the GSM recommendation 05.03, the efficiency of which canbe illustrated by a convolution coding ratio X/Y, which signifies that Xdata bits are represented in the channel coding by Y code bits. On afull-rate GSM traffic channel, on user data rates 9.6 kbit/s, 4.8 kbit/sand 2.4, the convolution coding ratios of 1/2 (punctured), 1/3 and 1/6,respectively, are employed.

The circuit-switched non-transparent connection between the TAF and theIWF on a GSM traffic channel comprises several protocol layers.

The terminal interface between the MS terminal adaptor TAF and the dataterminal equipment, as well as the interface between the IWF and e.g. anaudio modem MODEM are in accordance with CCITT V.24, and in FIG. 2 theterminal interface is marked with the symbol L2. As far as the inventionis concerned, the interesting protocols are L2R (Layer 2 Relay) and RLP(Radio Link Protocol) which both are at the terminal adaptor TAF and thenetwork adaptor IWF at both ends of the connection. In addition, theconnection has, as illustrated in FIG. 2, different kinds of RateAdaptation (RA) functions, such as RA1' between the TAF and a CCU unit(Channel Codec Unit) located at the BSS, RA1 between the CCU and theIWF, RAA between the CCU and a transcoder unit TRAU placed apart fromthe base station, and RA2 between the TRAU and the IWF. The rateadaptation RA functions are defined in the GSM recommendations 04.21 and08.20. Communication between the CCU and the TRAU is defined in the GSMrecommendation 08.60.

The information rate-adapted on the radio interface RA1' is furthermorechannel-coded the way specified in the GSM recommendation 5.03,illustrated by blocks FEC in the MS and CCU.

The present invention, however, only relates to the L2R/RLP operation ofthe TAF and the IWF, and communication between them. The otheraforementioned lower layer protocols, functions and units only provide atransmission path according to the GSM recommendations between L2R/RLPunits, and they are not significant to the present invention with theexception of channel coding FEC. Consequently, the other functions arenot described herein in any greater detail.

L2R (Layer 2 Relaying) functionality for non-transparentcharacter-oriented protocols is defined e.g. in the GSM recommendation07.02. L2R packs the user data and the status information originatingfrom the terminal interface into 200-bit, 25-octet long PDUs (ProtocolData Units), such as the one illustrated in FIG. 3. The octets arenumbered 0-24, octet 0 being transmitted first. The bits in the octetsare numbered 1-8, bit 1 being transmitted first. In a PDU, the octet maybe a status octet, a character (higher layer data) or fill bits. Octet 0is always a status octet. A status octet comprises 3 bits, SA, SB and Xfor the status of the V.24 connection, and 5 bits that indicate thenumber of data octets succeeding the status octet, as well as thespecial indications of the data octets such as empty and PDU. In FIG. 3,status octet 0 is succeeded by 3 data octets into which the word "GSM"has been packed, after which a new status octet 4 follows.

The L2R PDUs are packed in a frame according to the RLP protocol, such aframe being shown in FIG. 4. The RLP protocol is specified in the GSMrecommendation 04.22. The RLP is a balanced (HDLC type of) data transferprotocol with a frame structure, in which error correction is based onretransmitting corrupted frames at the request of the receiving party.The RLP extends from the mobile station MS terminal adaptor TAF to thenetwork adaptor IWF, which is usually located at the MSC. As shown byFIG. 4, the RLP frame structure comprises a header field (16 bits), aninformation field (200 bits), and a frame check sequence (24 bits). The200-bit L2R PDU is packed in the information field. As a result, the netRLP data rate is clearly above the maximum 9.6 kbit/s data rate for onechannel, which allows a specific number of retransmissions without adecline in the nominal user rate. For example, if the user rate on theterminal interface is 9600 bit/s and the data rate on the radiointerface is 12 kbit/s, the "surplus capacity" is, depending on thecharacter structure being used, at least 12.5%.

The transmission buffer buffers the data received from the V.24interface so that data will not be lost even if the MS is not able totransmit it instantly over the radio interface. A reception bufferbuffers data which is transferred to the V.24 interface so that datareceived from a traffic channel will not be lost even if it cannotimmediately be forwarded via the V.24 interface to e.g. the terminalequipment TE. The RLP protocol also includes a flow control, used toadjust the fill level of the transmission and reception buffers. Theflow control is specified in the GSM recommendation 07.02. The criterionused to activate the flow control may be a half full transmission orreception buffer.

The maximum user data rate on a single GSM traffic channel is restrictedto 9.6 kbit/s.

In high-speed data services (HSCSD), several traffic channels areassigned to a data call; in other words, two or more time slots are,assigned from the same TDMA frame. An example of how to carry outhigh-speed data transfer based on a multitude of traffic channels isdisclosed in the applicant's co-pending PCT application WO95/31878 andWO96/18248. However, it should be noted that as far as the invention isconcerned, the only significant matter is that a multichannel data linkcan be established.

FIG. 5 illustrates a GSM network architecture implementing such a datatransfer on several parallel traffic channels. FIG. 5 is similar to FIG.1, with the exception that in FIG. 5 there is, between the TAF and IWF,a circuit-switched non-transparent connection which consists of Nparallel traffic channels ch0-chn, where N=1,2,3 . . . . In the mobilestation, the TAF functions as a divider which splits the high-speed datasignal DATA IN originating from the data terminal equipment intoparallel traffic channels ch0-chn, and a combiner which combines thesubsignals received from the parallel traffic channels ch0-chn back intoa high-speed data signal DATA OUT. Correspondingly at the other end ofthe multichannel data link, the IWF functions as a divider which splitsthe input high-speed data signal DATA IN into parallel traffic channelsch0-chn, and a combiner which combines the subsignals received from theparallel traffic channels ch0-chn back into a high-speed data signalDATA OUT.

The protocol structure of FIG. 2 may also be applied to the FIG. 5multichannel connection architecture. In the preferred embodiment of theinvention, the L2R/RLP unit is common to all the traffic channelsallocated for the same connection. Out of these, the RLP at the sametime functions as the aforementioned divider and combiner. However, eachtraffic channel is provided with a dedicated rate adaptation (RA) andchannel coding (FEC) functions, as illustrated by FIG. 2. Thus, from theview point of the L2R/RLP unit, the multichannel data link issubstantially similar to a single channel link; the "transmissionchannel" between them only has more capacity than before.

The HSCSD connection can also be illustrated by the block diagram ofFIG. 6, in which the L2R and RLP protocol functions are distributed intoseparate units 62 and 63 in the TAF/MS and correspondingly into units 64and 65 in the IWF/MSC at the receiving end. Unit L2R packs the data inthe L2R PDUs as described above. However, in the HSCSD slightly modifiedPDUs are employed; the number of bits in the PDUs is smaller, i.e. 192.The reason for this is to compensate for the reducing user data rate inthe RLP protocol, the reduction being caused by the extra overheadinformation required by the HSCSD in the RLP frames. As to the presentinvention, the difference caused by the HSCSD in the PDUs and the RLPframes is not relevant and will not be described here in closer detail.The terms PDU and RLP in this application are used to refer to all theversions.

L2R unit 62 outputs the PDUs to RLP unit 63. RLP unit 63 inserts thePDUs in RLP frames and splits them in subchannels ch1-chn, where n is aninteger and ≧2. A radio transceiver TRx carries out for each subchannel(following rate adaptation and channel coding) building of bursts,interleaving and modulation, and transmission over the radio interfacein a corresponding subchannel time slot. The BTS radio transceiver TRxreceives the bursts in the subchannel time slots, and carries outdeinterleaving separately for each subchannel, and (following channeldecoding) forwards the RLP frames of each subchannel ch1-chn to RLP unit64 which combines the PDUs and feeds them to L2R unit 65. Between theBTS and IWF, the RLP frames have to be transferred in TRAU frames butthis is not significant to the present invention.

When the user data rate in a HSCSD service substantially correspondswith the maximum capacity allocated to the connection, full PDUs aretransmitted on all subchannels. If, as noted earlier, the user data ratein a HSCSD service is lower than the maximum capacity of the radio linkprotocol, L2R unit 62 may build partially filled PDUs and transmit themover the radio interface subchannels in RLP frames. Conventionally,however, the transmission has been distributed arbitrarily on all thesubchannels ch0-chn, with the result that all the subchannels are moreor less active.

According to the present invention, RLP unit 63 uses for transmittingRLP frames only a specific portion of the subchannels ch1-chn in casesthe maximum data transfer capacity allocated to the connection is notrequired. On the other, unused subchannels, there is no transmissionpresent at all, or they have a channel-specific DTX. The minimum numberof subchannels used is defined according to the actual data user rateinto the L2R unit, and the need for any additional channels is definedaccording to the buffered data amount. This will be described in theexemplary embodiment below with reference to FIG. 6.

The amount of input data, and the fill level of the buffers in the L2Runit 62 and RLP unit 63 are monitored by a DTX control unit 61. Atfirst, it is assumed that the maximum transfer capacity allocated to adata call is three traffic channels, i.e. 3×9.6 kbit/s=28.8 kbit/s. DTXcontrol 61 monitors the actual user data rate of the data flow into L2Runit 62. If the average user data rate exceeds the capacity of twosubchannels, i.e 19200 kbit/s, the minimum capacity required will bethree subchannels, i.e the same as the maximum capacity allocated to thedata link. If that is the case, DTX control 61 directs RLP unit 63 totransmit in the normal manner on all three subchannels.

It is further assumed that the average user data rate into L2R unit 62drops during the call to a value lower than 19.2 kbit/s, e.g. 18 kbit/s.DTX control 61 defines a minimum number of traffic channels for theaforementioned average user data rate of 18 kbit/s, i.e. twosubchannels. DTX control unit 61 thereby directs RLP unit 63 to activelytransmit on only two subchannels, for example on subchannels ch1 andch2. Transmission on the third subchannel ch3 will be temporarilyinterrupted or discontinuous transmission DTX according to the GSMspecifications will be activated on it. In such a case, RLP unit 63transmits, to the subchannel ch3, L2 fill frames according to the GSMrecommendation 04.06, item 5.4.2.3. The TRX receives these RLP framesfrom RLP unit 63, but only forwards them onto the radio path in specificTDMA frame subgroups, specified in the GSM recommendation 05.08, item8.3. At other times, the subchannel has no transmission in the DTX mode.

DTX control 61 continues to monitor the average user data rate, andsimultaneously monitors at least the fill level of the RLP unit 63buffer. The RLP buffer contains both the RLP frames to be transmittedfor the first time and RLP frames already transmitted to which noacknowledgement has yet been received from the receiving end. DTXcontrol 61 may have e.g. two threshold values for the buffer fill level.When the fill level of the RLP buffer is below a specific thresholdvalue, DTX control 61 considers the present number of subchannels to beadequate. If the buffer fills to this threshold value, DTX control 61considers the present number of subchannels to be inadequate for datatransmission. This leads to DTX control 61 deactivating DTX fromsubchannel ch3. RLP unit 63 begins to transmit user data in RLP frameson subchannel ch3, too, even though the minimum capacity correspondingto the average user data rate of 18 kbit/s is two subchannels. As aresult, the RLP unit 63 buffer begins to empty. As soon as the bufferempties down to the lower predetermined threshold value, DTX control 61considers the minimum capacity corresponding to the average user datarate, i.e two subchannels, to be adequate for transmission.Consequently, DTX control 61 reactivates DTX on subchannel ch3. RLP unit63 recommences transmitting RLP frames containing user data only viasubchannels ch1 and ch2. Via subchannel ch3, fill frames aretransmitted.

If the average user data rate drops to lower than 9.6 kbit/s, theminimum capacity required is one subchannel. In such a case, the DTXmode will also be activated e.g. on subchannel ch2, and user data isonly transmitted on subchannel ch1. DTX control 61 continuously monitorsthe buffer fill level of RLP unit 63, and if necessary deactivates DTXfrom one or more subchannels.

If the average user data rate again gains a value higher than 19.2kbit/s, DTX control 61 again takes all the allocated subchannels ch1-ch3in normal use.

Alternatively, the fill level of the buffer in RLP unit 63 may haveseveral threshold values in both directions. Upon filling of the bufferto a specific threshold value, DTX is deactivated from one or moresubchannels; upon reaching the next threshold value, DTX is againdeactivated from one or more subchannels, etc. In a similar manner, asthe buffer empties to a threshold value, DTX is reactivated on one ormore subchannels; as the next threshold values is reached, DTX is againactivated on one or more subchannels, etc.

At the receiving end, the BTS and the IWF operate as normal, receivingon all subchannels. DTX on specific subchannels is dealt with accordingto the GSM recommendations.

The invention is most advantageous when applied to a MS transmission.The method is also applicable when transmitting from the mobilecommunication network to the mobile station, whereby the radiointerference level in the mobile communication network will be lower.

As noted above, for determining the user data rate and monitoring thebuffer fill level, different kind of statistical methods can be applied.Also, as noted above, the traffic channels to be used at any one timemay be selected according to a specific preference list.

The figures and the explanation related thereto are only intended toillustrate the present invention. The method of the invention may varyin its details within the scope of the attached claims.

We claim:
 1. A method for high-speed data transfer in a digital mobilecommunication system, the method comprising the steps ofestablishing anon-transparent data connection having a number of parallel subchannelsallocated on the radio interface, said number being determined by aspecific maximum transfer capacity; receiving user data from a terminalinterface at a varying user data rate; transmitting user data over thenon-transparent data connection in data frames by employing acommunication protocol which acknowledges data frames received correctlyand retransmits defective data frames; buffering data frames to betransmitted in a transmission buffer; storing the data frame transmittedin the transmission buffer for a possible retransmission until anacknowledgement is received from the receiving end; determining theactual user data rate on the terminal interface; determining a minimumnumber of subchannels, said number being determined by the actual userdata rate; transmitting user data in data frames only via specificsubchannels corresponding in number to said minimum number ofsubchannels; interrupting transmission or activating discontinuoustransmission on each surplus subchannel allocated to the connection;monitoring fill level of the transmission buffer; continuingtransmission or deactivating discontinuous transmission on at least oneof said surplus subchannels if the transmission buffer fill levelreaches a first threshold value; and interrupting transmission orreactivating discontinuous transmission on at least one of said surplussubchannels if the transmission buffer fill level decreases to a secondthreshold level.
 2. A method as claimed in claim 1, comprising thefurther steps of:determining a minimum number of subchannels, saidnumber being determined by the actual user data rate, transmitting userdata in data frames via specific subchannels the number of which atleast equals said minimum number of subchannels, and changingdynamically the number of said allocated subchannels according to thefill level of the transmission buffer.
 3. A method as claimed in claim2, wherein the dynamic changing of the number of specific subchannelscomprises the steps of:continuing transmission or deactivatingdiscontinuous transmission on said at least one of the othersubchannels, if the transmission buffer fill level reaches a firstthreshold value, and interrupting transmission or reactivatingdiscontinuous transmission on said at least one of the othersubchannels, if the transmission buffer fill level decreases to a secondthreshold value.
 4. A method as claimed in claim 3, comprising thefurther step of:transmitting user data via all the allocated subchannelsif said number of subchannels matches the number of subchannelsallocated for the data connection.
 5. A method as claimed in claim 2,comprising by the further step of:transmitting user data via all theallocated subchannels if said minimum number of subchannels matches thenumber of said subchannels allocated for the data connection.
 6. Amethod as claimed in claim 1, comprising the further stepof:transmitting user data via all the allocated subchannels if saidminimum number of subchannels matches the number of subchannelsallocated for the data connection.
 7. A digital mobile communicationsystem comprisinga transmitting party having a transmission buffer; areceiving party; a multichannel non-transparent circuit-switched dataconnection between the two parties, said multichannel data connectionhaving parallel subchannels allocated on a radio interface, the numberof said allocated subchannels being determined by a specific maximumtransfer capacity; and a communication protocol in which data istransferred over said data connection in data frames so that data framesreceived correctly are acknowledged and defective data frames receivedare retransmitted, said transmission buffer buffering the data frames tobe transmitted and storing the transmitted data frames until it receivesan acknowledgement of successful reception, the transmitting party beingarranged to monitor the actual user data rate and the fill level of thetransmission buffer, and the transmitting party being arranged totransmit user data in data frames via specific allocated subchannels,the number of said allocated subchannels depending on the actual userdata rate and the fill level of the buffer, and to interrupttransmission or to activate discontinuous transmission on possible otherallocated subchannels.
 8. A system as claimed in claim 7, wherein thetransmitting party is a terminal adapter of the mobile station, and thereceiving party is a network adaptor of the mobile communicationnetwork.
 9. A system as claimed in claim 7, wherein the transmittingparty is a network adaptor of the mobile communication network, and thereceiving party is a terminal adaptor of the mobile station.
 10. Amethod for high-speed data transfer in a digital mobile communicationsystem, comprising the steps of:establishing a non-transparent dataconnection having a number of parallel subchannels allocated on theradio interface, said number being determined by a specific maximumtransfer capacity; transmitting user data over the non-transparent dataconnection in data frames by employing a communication protocol whichretransmits defective data frames; buffering data frames to betransmitted in a transmission buffer, transmitting user data in dataframes via specific one or ones of said allocated subchannels, when themaximum data transfer capacity allocated to the data connection is notrequired, interrupting transmission on the remaining one or ones of saidallocated subchannels which do not belong to said specific ones of saidallocated subchannels, if any.
 11. A digital mobile communication systemcomprisinga transmitting party having a transmission buffer; a receivingparty; a multichannel non-transparent data connection between the twoparties, said multichannel data connection having parallel subchannelsallocated on a radio interface, the number of said allocated subchannelsbeing determined by a specific maximum transfer capacity; acommunication protocol in which data is transferred over saidmultichannel data connection in data frames so that defective dataframes received are retransmitted, said transmission buffer bufferingthe data frames to be transmitted; the transmitting party being arrangedto transmit user data in data frames via a specific one or specific onesof said allocated subchannels, when the maximum data transfer capacityallocated to the data connection is not required, and to interrupttransmission on the remaining one or ones of said allocated subchannels,if any.
 12. A data transmitter for data transmission over a multichannelnon-transparent data connection in a digital mobile communicationsystem, said multichannel data connection having parallel subchannelsallocated on a radio interface, the number of said allocated subchannelsbeing determined by a specific maximum transfer capacity, and said datatransmission on said multichannel connection having a communicationprotocol in which data is transferred over said multichannel dataconnection in data frames so that defective data frames received areretransmitted, said data transmitter comprisinga transmission buffer forbuffering the data frames to be transmitted, said data transmitter beingarranged to transmit user data in data frames via a specific one orspecific ones of said allocated subchannels, when the maximum datatransfer capacity allocated to the data connection is not required, andto interrupt transmission on the remaining one or ones of said allocatedsubchannels, if any.